This is a weblog for those living in the Avonside area and and anyone else looking for information or news on earthquake assessment, rebuilding and related matters.

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Saturday, April 30, 2011

Respecting waterways and land.

On the 27th of April 2011 the Press carried an article featuring landscape architect Di Lucas (page A5). Entitled Rebuild must 'respect' waterways,
it discussed how parts of the inner city had been built on top of
tributaries to the Avon, creating hidden vulnerabilities. To quote from
the article: "When transposed on the present-day central city, the
waterways mirrored some of the areas of worst earthquake damage."

The article lists a few of the inner city streets where there had once
been streams, and which have now experienced significant land and
building damage. A map showing the inner city in the 1850s, and the
tributary streams, accompanies the article. An on-line version without
the map is here.

Focused as it was on the inner city, the article did not mention the
situation in the suburbs. We are most likely to find the same situation,
and consequences, occurring there. For those who want to look at this
on a broader scale, while waiting for the geotech reports in June or
later, there are two items worth tracking down.

Black Maps

The first to look at is a copy of the original "black maps", or early
maps derived from them (such as the one used in the Press article). The
black maps were published in 1850s, based on the work of early
surveyors. Unlike today's surveys, these maps do not contain micro
levels of detail. What they do provide is a good description of the
nature of the various areas within the city. Some areas have more
detail than others, which are only broadly described (e.g. between
Travis Road and Lower Styx Road the whole area is described as "swamp,
Manuka scrub fern, swamp hills, manuka scrub and fern, sand hills". The
Avonside part of the map was published on the blog in March, here.

The maps cover everything inside the area starting at the mouth of the
Waimakariri, over to the back of the airport, across to Hornby, Halswell
and the bottom of the Port Hills, through Cashmere, then follows the
course of the Heathcote River and the land on either side, to Opawa,
Ferrymead, the Estuary, Redcliffs and the Sumner bar.
The early maps don't give us a means for DIY geotech analysis, but do
show what the land was originally like. When coupled with EQC's geotech
results we can deal knowledgably with a few basic issues:

why some people do have to go

why some people should consider going instead of staying (or vice versa)

And ask the questions:

why were some developments or constructions allowed to proceed?

what developments and construction can be done safely throughout the city and the vulnerable suburbs?

A PDF map, based upon the black maps, is available from the CCC websitehere. The map can be enlarged quite significantly to examine just one small part of the city.

Swamp City

Christchurch - Swamp to City, published by the Christchurch
Drainage Board in 1989 (author John Wilson, ISBN 0908714041, copies at
most libraries) is a history of the Drainage Board from 1875 to 1989.

Accompanying the history of turning Christchurch from swamps to dry land
are interesting photographs of flooding in Christchurch at various
times, and some useful maps. The map facing page 16 shows the height of
the city using 1 metre contour lines. Its amazing just how low lying
the eastern suburbs were back then, and now even lower. The fold-out
map inside the back cover is a compilation from the original black maps
overlaid with streets. Identical, or very similar, to the PDF version
mentioned above.

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